The need to optimize the DC bias point and the amplitude of the drive signal has been recognized in various prior art articles, such as that illustrated and described in Electronics Letters, 23rd Nov., 1978, Volume 14, No. 24, on Page 775, by D. W. Smith, and a later article in the IEEE 13th International Conference on Circuits and Systems in 1980 entitled, "Laser Level Control For High Bit Rate Optical Fiber Systems" by D. W. Smith and T. G. Hodgkinson on Page 926. However, these prior art approaches could only maintain the drive signal in accordance with an initial setting or condition of the laser diode. In other words, as the laser changed its threshold knee, and/or slope of the light vs. current characteristic due to age, temperature and other factors, the referenced control systems would no longer provide optimum output information. As is known to those skilled in the art, the knee is the threshold of current through the diode at which the light output is uniform in frequency or in other words, the diode is in the laser mode.
As is known to those skilled in the art, and as brought out in these referenced articles where the data being optically output has a high bit rate, it is desirable that the logic zero portion of data bits drive the laser diode current to a value that is very close to the threshold point or knee of the LI curve of the diode. Thus, when a given drive signal and a given bias is applied to a diode and its characteristics change, the drive will normally extend considerably below the knee of the diode since, typically, with aging and higher temperatures, the knee of the curve occurs at greater currents and the slope changes such that much greater currents are required to provide the maximum light output. Since it is undesirable to overdrive the diode whereby the life is substantially shortened, the average power output needs to be controlled at the same time as the drive signal is controlled.